Antihalation coatings



f highly efficient antihalation coating.

Patented Ecifl 8,

AN TIHALATION COATINGS Emmett K. Carver, Rochester, N. Y., assignor to Eastman Kodak Company, Rochester, N. Y., a corporation of New Jersey Application February 17, 1942, Serial lNo. 431,260 l 6 Claims.

This invention relates tok photographic lm,

. and particularly to antihalation layers for such film.

It is an object of the invention to provide .a

One difiiculty encountered in the use' of anti- `jhalation layers is the fact that the removal thereof after exposure is not easy, particularly when the layer is a high density layer. For this reason low density antihalation layers are preferable, but, of course, they are not as efficient in reducing halation. Also, in those processes of photography which require exposure through the support for the emulsion, an antihalation layer is placed on the back of the support and must have a low density particularly to light of normal incidence. Of course, low optical density layers have relatively high density to oblique rays of light, and it is the oblique rays of light that are the most troublesome and cause the widest halation.

It is, therefore, an object of the invention to produce a layer to reduce halation at practically all obliquities and still have relatively low optical density, particularly at normal incidence.

It is an object of a preferred embodiment of 'the invention to have an antihalation layer which is highly efficient at high and medium obliquities of light, but has reduced density to light of normal incidence, which is harmless anyway as far as halation is concerned.

According to the invention, an optical interference reflection reducing layer is made to cooperate With an antihalation dye layer in a novel manner to give a highly efficient antihalation support particularly a support in which the falling olf of the efficiency of the dye at low obliquities is counteracted. Halation is caused by light striking a photographic emulsion layer and being transmitted or reflected diifusely and then being specularly reflected by the back of the support so as to strike the emulsion again to one side of the point at which it was originally focused. Of course, the diffusion of the light by the emulsion layer is not complete and, hence, there is a greater intensity of light in the region immediately aroundthe original point and in general, this is not objectionable since it falls above, for less oblique or normal incident light. Non-reflecting optical interference layers can, by proper selection of thickness, be made efficient at any specified angle, but the efiiciency falls off at other angles of incidence. According to the invention, the interference coating is selected to cooperate with the dye by being efficient at angles less than those at which the dyev is most eicient, i. e. at angles of 10W Obliquity. In the preferred embodiment of the invention the dye has` .a low optical density to perpendicularly incident light, i. e. density less than .5. Such a low density is permitted by the present invention, since the optical interference layer takes care of the lscattered light up to relatively high obliquities, after which even this low density layer becomes highly absorbent.

Inthe mostpreferred embodiment of the invention the interference layer has a thickness givingsubstantially optimum interference to light incident at angles between 10 and 60 from the normal. For normal light and light of less than l0 degrees spread, the halation is quite harmless since it falls on or Aimmediately adjacent to the point of the emulsion originally exposed. At angles greater than from the normal any dye layer which is used, even one of low normal optical density, is highly efficient. The present invention permits the use of even lower density dyes than previously permissible.

Although the interference layer should be on the back of the support in order to get the greatest efficiency, the dye may be in variousrpositions.k For example, if it is a low density dye, it

may be incorporated'right in the supportitself or thesupport may be made up of a plurality of layers in one of which the dye is incorporated. rIhe additional layer constituting part of the support may be immediately adjacent to the emulsion or may be between the interference layer and the support proper. Of course, if the dye layer is between' the interference layer and the Within the circle of'confusion of the optical system anyway. Any antihalation dye, whether of high or low density has its highest density to oblique light which, of course` is the most troublesome even though it is of reduced intensity. The dye also has less optical density, preferably quite low density in the embodiment referred to coating is applied thereto.

support proper, it may be thought of as constituting part of the( antihalation coating on the backof the support. Additional dye may also be included in the interference layer itself.

This invention may be combined with that'described in U. S. application 358,512, Nadeau and Hilborn, filed September 26,1940, now U. S. Pat- 4ent`2,331,7l6, wherein a high index layer is diffused into the support, before the interference As before, the dye may be incorporated in the support, or in the inbetween layer which in this case has high index of refraction. A particularly satisfactory ari rangement results when the dye is. incorporated strikes.-zthe.-same.r point in` the emulsion as thatv .n

tical interference layersV involving a plurality oi?` layers are equally useful withrthe present invention which does not depend on the type of interference layer used;

'I'he various embodiments of the invention. will be more fully understood fromv the description'of the accompanying drawing inwhich:

Fig- 1 shows a simple embodiment of the-invention. Fig.' 2y shows a preferredembodi'ment in'which the interference layer has optimum elect at angles of incidence somewhat different'from normal.

Fig. 3 shows a similar embodiment wherein the emulsion is exposed .through the base.

f Fig. 4 illustrates the. principle of the inventionthe thickness of the interference coating being exaggerated.`

Fig.15 similarly showsa combination of the present invention with that of Nadeau et al. mentioned above.

' Fig: 6l similarly shows an embodiment wherein the'dyer is incorporated ina layer immediately under they emulsion.

Fig. 'Ilsimilarly shows an embodiment'where-v inxthe dye is incorporated in the base itself.'

In Fig. `l lm support I isv coated on one `side viwlthfanemulsion I I and on theother sidewith 1an antihalation backing'IZ with anjoptical inter- 1 ference coating on the exposedsurfacethereohy Lightrepresented'bythe arrowl YI3 striking the j emulsion is scatteredby particle I4 of,l the `emul-` sion so that someA of it travels' straight through.

ther antihalation backing obliquely. Since this latter layer I2 has. a surface treated tocauseI optical interference,v the ray I5 is onlyslightly l reilected, if at all, and thev major portion! of it' passes through as shownl bythe arrow I6. On the other hand, the oblique ray I1 striking the normally incident ray, but since. the path .of the light through the antihalation backing,'i. e.V since the distance between the points I8, I9, `and, 2Il is. relatively long, the dye in this layerfa'bsorbs much of the light from the ray I1 sothatonly a very small part'thereof as represented by the arrow 2| is reflected to cause halation in the emulsion. ,InfFig 2, the interference coating on thefanti' halationY` layer 25 is selected of such thickness that it has its optimum effect for light incident thereon at angles between 10 and v60" from the normal. In this case, the normal ray 26 is partly reected as shown by thexarrow 21, but the slightly oblique ray 28 is practically entirely` transmitted as shown by the arrow 29 and-only slightly reflected, if at all, as shown by the broken line 30. The advantage 'of thisvembodiment is the fact that the ynormally, incident light when reflected is entirely harmless anyway since it i interference coating would not be .reduced in -ref-j ilection by interference `as much as would-the to permit wider selectioniof materials for layer. during at which it was transmitted. Therefore, thelight represented-by thearrow 2l'isharm1ess. Also. since the interference coating is quite efllcient forraysbetween l0 .and\60 from the normal, .it ismore eicient than the coating-,on layerIZ of Fig.. 1 for rays of'medium Obliquity, e. g.' rays of from the normal. Thus, in this case the interfercnce coating on layer 25kr cooperates more closelywith the effect of the dye and inturn permitsa dye of lower density kto beused.

In Fig. 3 the light. 35'from the subject'or lens. passes through'the antihalation backing 25 as. shown by the ray 36 tostriketheemulsion Il .and i the .scattering is by4 reflection. In this cas'ei4 the ray 38l is transmitted centage thereofis reflected to c ause halation', as indicated s by` the arrow. 39.

In. Fig. 4 a dye layer "40 rvisr appliedto thebase v I0 before the interference layer 4 I'is placed thereon.. The thicknessjof the coating. -is greatly exaggerated in this gure to illustrate theiprinciple thereof, but the present invention vis independent of thefmethod of producing theoptical interferencelayer ontheydye layer.' The inter ference layer may have itsxoptimum absorption either for normal light corresponding vto Flg.{1 orl for slightlyk obliquelight correspondingv to Fig. 2. In either case the normally incident 'light ,I5 is mostly transmitted asshown bythe arrow 42.

.The light reflectedjfrom the'front and backsurface of the interferencelayer passes throughy av long path in the dye layer 40 and only a small amountiof light as indicated by arrows 43and 44` Obviously, the.

reachesthe emulsion .layerf I I. layer 40 `may lbe consideredalong with the layer II) 'as constituting. the'support, 'orthe layers 40y and `4 I may vbe considered .as `combined land cons'tituting the..antihala`.t ic' n coating vfor: asimple as shown'by the arrows I5 and `IIiwhereas some of it is scattered vas shown by arrow vI'I. to lstrike support I0 lasinFig'. l.

, In Fig.l 5 the inventionis combined with that of Nadeau and Hilborn mentioned above, and in this case ahigh index layer -is diiusedinto the support I0 as indicated by the `diffused interface 5I. In accordance with the teachings of Nadeau and Hilborn such ahigh index layer permits a more efficient interference layer 52 `to be coated thereon. As before, this layer may becoated to have its optimum eiiiciency eitherf'or normal! l lightlor.for'slightlyoblique light.y The normal' ray I5z'is atleast morehighly'transmitted.thanit would be in'the `absencexof. the interference layer, as' indicated. by` the: arrow- 55; The .reilec'f tion fromthe front andzbackfsurfaces oflthe interfercnce layery 52..i s vindicated by arrows 53,and v :54 andhas.. reduced vintensity ldue .to the dye is vv` incorporated. either in thela'yer IIl'or in the layer 50. Mostpreferably, .the dye is .infthe vlayer 5U 50 and tozbe fairly accessible for removal processing. of.. the hlm;

In Fig. 6 the dye layerfis coated between.-

the base `II) andthe emulsion` I I andthe inter ference layer 6I is coatedon the back of the layer I0. As before, the normal ray I5 iis .mostly transmitted as shown by arrow 42 and the oblique ray4 I1 as reflected at the various interfaces is absorbed by the dye in the layer 60. If the layer and the layer Ifhave the same index of refraction, there is, of course, no reflection at the. interface there-betweenand, hence, the

by the 'interference coating on thelayer 25isince it strikes.` itatthe;angletofr 'optimum' interferenceeffct and the fray. is of such` great lObliquity that 'it is" greatly 'absorbed by the dye inthe laye'r125 and .only a" small .per-f layer, and an optical interference layer onvthe y ray 63 has zero intensity. However, both the rays 64 and 65 are absorbed by the dye in the layer 60 asis the ray l1 itself when first transmitted therethrough. l

In Fig. 7 there is shown a simple form of the invention which is applicable to those embodiments requiring only a low density of dye. In this embodiment the base 10 itself contains the absorbent dye and the interference coating 1l is coated directly on this base. The normal ray l5 is mostly transmitted as shown by the arrow 42, the slightly oblique ray 16 is practically entirely transmitted as shown by the ray 11, and the highly oblique ray 13 is greatly absorbed in its first passage through the base 10, and after reflection as shown by the arrows 13 and 14 it passes again through the base to be again greatly absorbed before striking the emulsion layer to cause little if any halation.

Just as Figs. 2 and 3 correspond to each other, the embodiments shownl in Figs. 1, 4, 5, 6, and 7 may be used for exposure through the base when the antihalation dye used has a low density 'to normally incident light, i. e. has a density less than .5, preferably less than .3. Obviously, one of the main advantages of the interference coating is that it permits a less dense dye to be used. This is particularly useful when very low density dye is made necessary by the fact that the film must be exposed through the support. However, it is also useful where higher concentrations of dye are permitted, since it is always desirable to keep the concentration as low as possible to permit removal thereof during processing. On the other hand, interference layers alone are not entirely satisfactory since they have their optimum efficiency for one Obliquity only and fall off in efliciency at other angles of incidence. The combination of an interference eoat.

ing and a dye layer permits the dye to' take care back of the ysupport: for reducing the reflection of all halation producing light incident on the optical interference layer at angles less than 60 from the normal, said support and layer having an optical density less than .5to light normally incident thereon and sufficiently high to absorb eiectively all halation producing light obliquely incident thereon at angles greater than 60, said halation vproducing light in both'instances being that to which the photosensitive layer is sensitive.

2. A photographic film according to claim 1 in which the interference layer has a thickness giving substantially optimum interference to normally incident light.

3. A photographic lm according to claim l in which ther interference layer has athickness giving substantially optimum interference to light incident at angles between 10 and 60 from the` normal.

4. A photographic film' comprising a photosen- A v sitive layer-,a transparent support for the layer andan antihalation coating on the backofthe support having an optical density less than .5 to light normally incident thereon and sufi'iciently high to absorb all halation` producing light obliquely incident on the coating at vangles greater than 60 and including at least one layer of optical interference thickness for reducing the ref flection of all halation producing light less ob-v.

liquely incidentthereon, said halation producing light in both instances being that to which the photosensitive layer is sensitive.

5. A photographic fihn according to /claim 4 in which the coating includes between the interference layer Aand the support an intermediate layer containing a lightabsorbent having subof the highly oblique rays which in many cases are quite appreciablev even though they are of relatively low intensity and the interference layer to take care of the medium and/or low obliquityl ference layer and the support an intermediatev ylayer forming al sharp-interface with the 'interstantially all lof said optical density.`

' 6. A photographic film according to claim 4in which the coating includes between the interference layer and a diffused interface' with the What I claim and desire to secure by Letters Patent of the United States is:

1. A photographic film comprising a photosensitive layer, a transparent support for the support, said intermediate layer Vcontaining a light absorbent having substantially all of said f optical density. 

